1. Field of the Invention
The present invention relates to a speaker with an edge having advantageous characteristics, which is suitable for reproducing an audio signal of a large input.
2. Description of the Related Art
In recent years, a small size sound reproduction device which does not require a large space has been provided for a stereo system or a personal computer to be used at home. A speaker used for such sound reproduction device typically has a diaphragm of a small diameter (hereinafter, referred to as diaphragm diameter).
FIG. 9A is a cross-sectional view of an exemplary structure of a conventional speaker having a small diaphragm diameter. FIG. 9B is an enlarged cross-sectional view illustrating an edge 9 and the vicinity thereof in FIG. 9A. In FIG. 9A, an annular magnetic circuit including a center pole 2, a magnet 3 and a top plate 4 is formed at a lower end of an annular frame 1.
A magnetic flux of a high density is generated in an annular gap 5 formed between an outer periphery of an upper portion of the center pole 2 and an inner periphery of the top plate 4. A voice coil bobbin 6 is retained in the gap 5 in such a way that the voice coil bobbin 6 can vibrate freely in upward and downward directions. A voice coil 10 is wound around the periphery of the voice coil bobbin 6 at a lower portion thereof. When a driving current corresponding to an audio signal is applied to the voice coil 10, an electromagnetic force is generated in the voice coil 10, resulting in a piston-like vibration of the voice coil bobbin 6 in the electric field generated in the gap 5.
A diaphragm 8 is fixed at an upper end of the voice coil bobbin 6, and a damper (also referred to as a suspension) 7 is connected in vicinity of the upper end of the voice coil bobbin 6. The diaphragm 8 is attached to the frame 1 via the edge 9, while the damper 7 is attached to the frame 1 directly. The diaphragm 8 is retained directly by the edge 9 and indirectly by the damper 7, in such a manner that the diaphragm 8 is permitted to vibrate freely.
In the speaker thus structured, when a driving current proportional to the audio signal is applied to the voice coil 10, an electromagnetic force of the voice coil 10 and a magnetic flux of the gap 5 interact with each other, thereby generating a driving force in the voice coil 10, which in turn vibrates the voice coil 10. As the voice coil 10 vibrates, the diaphragm 8 retained by the damper 7 and the edge 9 is vibrated in upward and downward directions, so as to output a sound from the speaker.
As the edge 9 of the speaker, the edge shown in FIGS. 9A and 9B is most commonly used. The edge 9 is referred to as a roll edge since its cross-sectional configuration shows a shape of a semi-circular roll, and the edge 9 is disposed at the outer periphery of and concentrically with the diaphragm 8. As the material of the edge 9, a cloth impregnated with resin, a urethane foam sheet, a rubber sheet orthelike can be used. The roll-shapededge 9 and the wave-shaped (also referred to as a corrugation) damper 7 constitute a supporting system of the diaphragm 8 of the speaker, assuring a large vibration amplitude of the diaphragm 8.
However, a diameter of the diaphragm 8 in the conventional small speaker as shown in FIG. 9A is too small to generate a large sound pressure. In other words, in order to acquire a particular magnitude of sound pressure, the vibration amplitude of the diaphragm 8 needs to be increased in an inverse proportional manner with respect to an area of the diaphragm and the square of a frequency of audio signal.
The highest amplitude of the diaphragm is proportional to a size of the roll of the edge 9. There is no significant difference between the size of the roll in a speaker with a small diaphragm diameter and that of the roll in a speaker with a large diaphragm diameter. Therefore, obtaining a sufficiently large amplitude in a speaker with a small diaphragm diameter is difficult, and thus a problem remains that a reproduced sound deteriorates especially in a low frequency range.
FIG. 10 is a schematic cross-sectional view illustrating a displacement of the roll structure of the edge 9. In FIG. 10, a dashed line shows a state of the edge 9 where a driving current is not applied to the voice coil 10 and the diaphragm 8 is at a neutral position. In contrast, a solid line shows a state of the edge 9 where a large driving current of a low frequency is applied to the voice coil 10 and the diaphragm 8 is displaced along the Z axis in a (+) direction. As is clear from FIG. 10, when a large driving current of a low frequency is applied to the voice coil 10, the edge 9 is extended to be stretched completely.
FIG. 11 illustrates a displacement of the edge 9, measured by a laser Doppler displacement analyzer, in one cycle during which a driving current corresponding to a sound of a drum being attacked is applied to the voice coil 10 and the diaphragm 8 is vibrated at an amplitude within a range of .+-.10 mm along Z axis. As is clear from FIG. 11, while the edge 9 shows a roll shape when the diaphragm 8 is at a neutral position, the edge 9 is displaced into a straight shape and stretched when the diaphragm 8 vibrates at the amplitude of .+-.10 mm.
When a large driving current of a low frequency is applied to the voice coil 10, the edge 9 is stretched, and an unusual sound is produced from the edge 9 so as to remarkably deteriorate the quality of a reproduced sound. The unusual sound is the same as a sound generated when a thin sheet of rubber, a cloth, a sheet of paper or the like Is suddenly stretched from a loose state (stretching sound).